In an integrated semiconductor structure, there are wells in which components or circuit units which have to be electrically insulated from one another are formed. A widely used insulation technique consists of the creation within a "chip" of semiconductor material of insulation regions with the opposite type of conductivity to that of the semiconductor material. During the operation of these integrated structures, the semiconductor material of the chip is brought to a potential relative to the insulation regions, such that the P-N junctions created between the semiconductor material and the insulation regions are reverse biased; the reversed polarization of these junctions ensures the insulation between the regions in which the various components or circuit units are formed.
In an integrated semiconductor structure comprising two or more regions formed with the same type of doping (homologous regions) within a chip of semiconductor material having doping of the opposite type, undesired circulation of current may occur, causing anomalous operation of the integrated structure. These currents are caused by the direct polarization of the P-N junctions described above which normally have reversed polarities. In particular, in a situation of this type, a lateral parasitic transistor is activated between two adjacent homologous regions with the consequent injection of current from the region which acts as the emitter to the region which acts as the collector. This current also switches on any vertical parasitic transistors present in the region which acts as a collector.
For example, the integrated structure shown in FIG. 1a will be considered. For simplicity of description, reference will be made below to P-type regions formed in an N-type semiconductor material; naturally, corresponding considerations are applicable if the regions have the opposite doping. The integrated structure shown in the drawing comprises an N-type epitaxial layer 110 formed on a substrate, not shown in the drawing. Two P-type regions 120 and 130 are formed within the epitaxial layer 110; and within the region 130 there is a further N-type region 140. During normal operation of the structure, the substrate, and hence the epitaxial layer 110, is kept at a positive potential relative to the potential of the regions 120 and 130. The P-N junctions created between the substrate 110 and the regions 120 and 130 therefore have reversed polarities, that is, polarities which ensure insulation between the two regions.
A typical example is that in which the integrated structure described above is a mixed-type integrated circuit, that is, with insulated signal-processing circuits in one or more wells and with vertical power transistors having their collector (or drain) regions in the epitaxial layer 110. It is assumed that the region 120 includes the anode of a recirculating diode associated with a vertical power transistor, not shown, integrated in the same circuit and having an inductive load. The region 130, on the other hand, includes the insulation region of a well. The region 140 includes the signal-processing components of the control circuitry of the power transistor.
In a situation of transitory or anomalous polarization of the circuit, the epitaxial layer 110 may be at a negative potential relative to that of the region 120, that is, the recirculation diode may be polarized for transmission. To examine the effect of this situation, it can be seen that the two regions 120 and 130 form a parasitic component with the substrate, as shown schematically in the equivalent circuit shown in FIG. 1b. In particular, the parasitic component T.sub.1 is a lateral PNP transistor created by the P region 120 (emitter), by the N epitaxial layer 110 (base) and by the P region 130 (collector). Moreover, in the example illustrated, a further parasitic component is constituted by a vertical NPN transistor Tv created by the N region 140 (collector), by the P region 130 (base) and by the N epitaxial layer 110 (emitter). As shown in the drawing, the collector of the transistor T.sub.1 is connected to the base of the transistor Tv (common region 130) and the emitter of the transistor Tv is connected to the base of the transistor T.sub.1 (common epitaxial layer). In the situation described above, the PNP transistor T.sub.1 becomes conductive, causing an injection of current, the collector current of the transistor T.sub.1, from the region 120 to the region 130. In the example shown in the drawing, this injection of current also switches on the vertical parasitic NPN transistor Tv.
It is known that, in a generic transistor, if the base, collector and emitter currents are indicated Ib, Ic and Ie, respectively and its current gain is indicated .beta. then, then: EQU Ic=.beta..multidot.Ib
Since Ie=Ib+Ic the following can be deduced, EQU Ic=.beta.(Ie-Ic); Ic=.beta..multidot.Ie-.beta..multidot.Ic; (1+.beta.).multidot.Ic=.beta..multidot.Ie
from which: ##EQU1## The current gain and the base, collector and emitter currents, respectively, of a generic transistor Tn are indicated below by the symbols .beta.n, Ibn, Icn and Ien.
The current injected from the region 120 into the region 130, which is the collector current Icl of the transistor T.sub.1, is: ##EQU2##
This current Icl is equal to the current Ibv injected into the base of the transistor Tv.
Various solutions are known in the art for minimizing the effects of the parasitic transistors described above. It will be noted that all of the known solutions address the problem of lateral parasitic transistors and that of any vertical parasitic transistors separately. As far as the lateral parasitic transistor between the two P regions is concerned, a first solution consists of suitable spacing of the two regions; however, this solution involves a considerable waste of useful area of the integrated structure.
A different solution is that of increased doping of the base of the parasitic transistor, that is, the portion of the epitaxial layer disposed between the two P regions, by means of a suitable region with a high concentration of N-type impurities. The gain of the lateral parasitic PNP transistor is thus reduced; since, as shown above, the current injected into the P region 130 is proportional to the gain of this transistor, the injection of current is thus reduced. If the substrate (and hence the epitaxial layer 110) has to withstand high voltages (as in the mixed integrated circuit described, in which the substrate comprises the collector of the power transistor) this highly doped N-type region brings the upper surface of the chip to a potential close to that of the collector electrode of the power transistor. Since this potential may be very high in comparison with the surface potentials of the adjacent P regions, a strong surface electric field may be formed. To reduce the effects of the surface electrical field thus created, an adequate structure for terminating the homologous P-type regions in the regions with high concentrations of N-type impurities is required, with a consequent considerable waste of area of the chip.
A further solution, shown in FIG. 2a, consists of the formation of a further P-type region 210 (a "channel stopper") in the epitaxial layer 110 disposed between the regions 120 and 130, that is, in the base region of the lateral parasitic transistor. This region 210 splits the parasitic component between the two regions 120 and 130 into two lateral parasitic transistors Tl1 and Tl2, as shown schematically in the equivalent circuit of FIG. 2b. In particular, the lateral parasitic PNP transistor Tl1 is created by the P region 120 (emitter), by the N epitaxial layer 110 (base) and by the further P-type region 210 (collector), and the lateral parasitic PNP transistor Tl2 is created by the P-type region 210 (emitter), by the N epitaxial layer 110 (base) and by the region 130 (collector). As shown in the drawing, the two transistors Tl1 and Tl2 have a common base (the epitaxial layer 110). Moreover, the collector of the transistor Tl1 is connected to the emitter of the transistor Tl2 (common P-type region 210). As in the previous case, the collector of the transistor Tl2 is connected to the base of the transistor Tv (common P-type region 130) and the emitter of the transistor Tv is connected to the bases of the two transistors Tl1 and Tl2 (common epitaxial layer 110).
In this integrated structure, in a situation of transitory or anomalous polarization, one of the two parasitic transistors Tl1 and Tl2 is brought to a high level of saturation with a consequent reduction in its gain and hence in the injection of current into the region 130. In particular, the current injected from the region 120 into the region 130, which is equal to the base current Ibv of the transistor Tv, is given by the collector current Icl2 of the transistor Tl2: ##EQU3##
Since the emitter current Iel2 of the transistor Tl2 is equal to the collector current Icl1 of the transistor Tl1 the following is deduced: ##EQU4##
Another solution shown in FIG. 3a, consists of the electrical connection of the P-type region 210 to the epitaxial layer 110 by means of a surface metal contact strip 310. To favor good ohmic contact with the surface of the epitaxial layer 110, in the example shown in the drawing there is also a region 320 with a high N-type concentration which prevents the formation of a PN junction between the N epitaxial layer 110 and the metal strip 310 (generally aluminum) containing P-type impurities. As shown in FIG. 3b, the emitter of the transistor Tl2 (P-type region 210) is short-circuited to its own base (the epitaxial layer 110) by means of the metal strip 310. The transistor Tl2 is therefore always cut off so that the injection of current between the two regions 120 and 130 is completely nullified. As described above, if the substrate (and therefore the epitaxial layer) has to withstand high voltages, this solution also requires an adequate termination structure resulting in the occupation of considerable area.
To reduce the effects of any vertical parasitic transistor, such as the transistor Tv, on the other hand, a dynamic polarization circuit is used and is constituted by a vertical transistor of the same type as the parasitic transistor the switching-on of which is to be avoided. As shown in FIG. 4a, a further P-type region 410 is created in the epitaxial layer 110 and an N-type region 420 is formed therein. The N-type region 420 is connected electrically to the region 130 by means of a surface metal contact strip 430.
The circuit equivalent to the structure described above is shown in FIG. 4b. As described above, the circuit shown in the drawing comprises the vertical parasitic NPN transistor Tv created by the N-type region 140 (collector), by the P-type region 130 (base) and by the N epitaxial layer 110 (emitter). The dynamic polarization circuit is constituted by a vertical NPN transistor Tp constituted by the N-type region 420 (collector), by the P-type region 410 (base) and by the N-type epitaxial layer 110 (emitter). As shown in the drawing, the emitter of the transistor Tp is connected to the emitter of the transistor Tv (common epitaxial layer 110) and the collector of the transistor Tp (N-type region 420) is connected to the base of the transistor Tv (region 130) by means of the metal connection 430. The circuit shown in the drawing comprises a further lateral parasitic transistor Tl', constituted by the P-type region 130 (collector) by the N-type epitaxial layer 110 (base) and by the P-type region 410 (emitter). The collector of the transistor Tl' is connected to the base of the transistor Tv (common region 130) and to the collector of the transistor Tp (by means of the metal connection 430), the emitter of the transistor Tl' is connected to the base of the transistor Tp (common P-type region 410), and the base of the transistor Tl is connected to the emitters of the transistors Tv and Tp (common epitaxial layer 110).
The transistor Tp is supplied by the supply system of the integrated structure by means of suitable driving circuitry (not shown in the drawing). As soon as the substrate potential of the structure (that is, the potential of the epitaxial layer 110) falls below a predetermined threshold (higher than that necessary to make the vertical parasitic transistor Tv conductive) a driver stage constituted, for example, by a constant-current generator (not shown in the drawing) is activated and supplies to the base terminal of the transistor Tp a current of a value such as to bring the transistor to saturation. The base and the emitter of the transistor Tv are therefore kept at the same potential, thus preventing the vertical parasitic transistor from being switched on.
It should be noted that, since the current supplied to the base terminal of the transistor Tp from the supply system is of limited value, this transistor cannot be saturated if the voltage difference between the substrate (and hence the epitaxial layer 110) and the region 130 assumes high negative values, because of the strong activation of the parasitic transistors which inject current into the region 130. This is particularly critical when the integrated structure is constituted by a mixed circuit (signal and power) comprising a vertical power transistor having its own collector included in the weakly doped N-type epitaxial layer 110. The emitter is constituted by a region with strong N-type doping formed within a P-type region which constitutes the base of the same transistor. In this case, the transistor Tp is normally formed by the same process which is used for the formation of the power transistor described above. It is therefore constituted by an emitter region with a low level of doping (epitaxial layer 110) and by a collector region with a high level of doping (region 420) so that it has a very low current gain.
From an observation of the equivalent circuit shown in FIG. 4b it can also be seen that some of the current supplied to the base terminal of the transistor Tp is lost because of the lateral parasitic transistor Tl'. As well as being taken away from the transistor Tp, this current is injected into the region 130. It therefore has to be extracted from the region 130 (by means of the transistor Tp), together with the current injected therein from the P region 120 by means of the lateral parasitic transistor Tl formed between the two P regions 120 and 130, as in the previous case.